Gravity Gradiometer

ABSTRACT

A gravity gradiometer is disclosed which has a sensor in the form of bars ( 41  and  42 ) which are supported on a mounting ( 5 ) which has a first mount section ( 10 ) and a second mount section ( 20 ). A first flexure web ( 33 ) pivotally couples the first and second mount sections about a first axis. The second mount has a first part ( 25 ), a second part ( 26 ) and a third part ( 27 ). The parts ( 25  and  26 ) are connected by a second flexure web ( 37 ) and the parts ( 26  and  27 ) are connected by a third flexure web ( 35 ). The bars ( 41  and  42 ) are located in housings ( 45  and  47 ) and form a monolithic structure with the housings ( 45  and  47 ) respectively. The housings ( 45  and  47 ) are connected to opposite sides of the second mount section  20 . The bars ( 41  and  42 ) are connected to their respective housings by flexure webs ( 59 ). Transducers ( 71 ) are located in proximity to the bars for detecting movement of the bars to in turn enable the gravitational gradient tensor to be measured. First and second feedback controllers are provided for monitoring disturbances of an external platform and an internal platform which mount the gradiometer for producing forces to stabilise the sensor.

FIELD OF THE INVENTION

This invention relates to a gravity gradiometer, and in particular, butnot exclusively, to a gravity gradiometer for airborne use. Theinvention has particular application for measuring diagonal andoff-diagonal components of the gravitational gradient tensor.

BACKGROUND OF THE INVENTION

Gravimeters are widely used in geological exploration to measure thefirst derivatives of the earth's gravitational field. Whilst someadvances have been made in developing gravimeters which can measure thefirst derivatives of the earth's gravitational field because of thedifficulty in distinguishing spatial variations of the field fromtemporal fluctuations of accelerations of a moving vehicle, thesemeasurements can usually be made to sufficient precision for usefulexploration only with land-based stationary instruments.

Gravity gradiometers (as distinct from gravimeters) are used to measurethe second derivative of the gravitational field and use a sensor whichis required to measure the differences between gravitational forces downto one part in 10¹² of normal gravity.

Typically such devices have been used to attempt to locate deposits suchas ore deposits including iron ore and geological structures bearinghydrocarbons.

International publication WO 90/07131 partly owned by the presentapplicants associated company discloses a gravity gradiometer. Thegradiometer includes a gimbal bearing arrangement comprised of threeconcentric rings in which is mounted the sensing equipment. The sensingequipment generally comprises two spaced apart bars respectively locatedin shielded housings and each mounted on a web bearing. The instrumentdisclosed in that application is relatively complicated in that itincludes a large number of parts and is relatively heavy which is adisadvantage particularly in airborne applications.

SUMMARY OF THE INVENTION

The invention provides a gravity gradiometer for measuring components ofthe gravity gradient tensor, comprising:

-   -   an external platform for mounting a Dewar;    -   a sensor arranged in the Dewar for cryogenic operation for        measuring the components of the gravity gradient tensor;    -   an internal mounting for mounting the sensor in the Dewar;    -   a first feedback controller for monitoring a disturbance of the        external platform and producing a force to counteract movement        of the external platform to stabilise the external platform; and    -   an internal feedback controller for monitoring a disturbance of        the internal platform caused by any movement of the external        platform after stabilisation of the external platform to        counteract movement of the internal platform and stabilise the        sensor.

Thus, any movement of the external platform which is not fullycounteracted and stabilised by the first feedback control and which doespass to the internal platform, can then be addressed by the secondfeedback controller to stabilise the internal platform to in turnstabilise the sensor.

Preferably movement of the internal platform caused by the disturbanceis sensed by linear and angular accelerometers to provide feedbacksignals to the controller.

Preferably the internal mounting comprises a mounting for mounting thesensor for movement relative to three orthogonal axes and the controlleris for supplying signals to actuators for moving the mounting about anyone or more of the three orthogonal axes to stabilise the mounting andtherefore the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention would be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a gradiometer of one embodiment of theinvention.

FIG. 2 is a perspective view of a first mount forming part of a mountingof the gradiometer of the preferred embodiment;

FIG. 3 is a view of a second mount of the mounting;

FIG. 4 is a view from underneath the mount of FIG. 3;

FIG. 5 is a cross-sectional view along the line IV-IV of FIG. 3;

FIG. 6 is a cross-sectional view along the line V-V of FIG. 3;

FIG. 7 is a view of the assembled structure;

FIG. 8 is a view showing the sensor mounted on the gimbal structure;

FIG. 9 is a plan view of a bar of the preferred embodiment;

FIG. 10 is a diagram showing actuator control;

FIG. 11 is a block diagram showing operation of the rotatable supportsystem;

FIG. 12 is a view of a gradiometer of the preferred embodiment;

FIG. 13 is a view of a first mount of a second embodiment;

FIG. 14 is a view of part of the mounting of FIG. 13 to illustrate thelocation and extent of the flexural web of the first mount;

FIG. 15 is a view of the mounting of FIG. 13 from beneath;

FIG. 16 is a view of the mounting of FIG. 13 including a second mount ofthe second embodiment;

FIG. 17 is a cross-sectional view through the assembly shown in FIG. 16;

FIG. 18 is a view from beneath of the section shown in FIG. 17;

FIG. 19 is a view from beneath of the second mount of the secondembodiment;

FIG. 20 is a view of the second mount of FIG. 19 from above;

FIG. 21 is an exploded view of the second mount of the secondembodiment;

FIG. 22 is view of the assembled mounting and sensors according to thesecond embodiment;

FIG. 23 is a perspective view of the gradiometer with some of the outervacuum container removed;

FIG. 24 is a plan view of a housing for supporting a bar according to afurther embodiment of the invention;

FIG. 25 is a more detailed view of part of the housing of FIG. 24;

FIG. 26 is a view of a transducer used in the preferred embodiment;

FIG. 27 is a view similar to FIG. 25 but showing the transducer of FIG.26 in place;

FIG. 28 is a diagram to assist explanation of the circuits of FIGS. 29and 30;

FIG. 29 is a circuit diagram relating to the preferred embodiment of theinvention, particularly showing use of one of the sensors as an angularaccelerometer;

FIG. 30 is a frequency tuning circuit;

FIG. 31 is a cross-sectional view through an actuator according to oneembodiment of the invention;

FIG. 32 is a view of part of the actuator of FIG. 31;

FIG. 33 is a diagram illustrating balancing of the sensors of thegradiometer of the preferred embodiment; and

FIG. 34 is a circuit diagram of a calibration sensor used when balancingthe gradiometer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic view of a gravity gradiometer according to thepreferred embodiment of the invention.

The gradiometer shown in FIG. 1 comprises a double walled Dewar 1 whichis supported in an external platform 2. The external platform 2 enablesadjustment of the Dewar and therefore the contents of the Dewar aboutthree orthogonal axes. The external platform 2 is generally known andits adjustment by suitable motors or the like is also known. Thus, adetailed description will not be provided.

A vacuum canister 3 is provided in the Dewar and the Dewar is suppliedwith liquid gas such as liquid helium He so that the gradiometer canoperate at cryogenic temperature. The Dewar 1 is closed by an end plate4 which includes connectors 5 a for connecting electrical leads (notshown) to external components (not shown).

The canister 3 is closed by an end plate 9 which includes connectors 5 bfor connecting electric leads (not shown) to the connectors 5 a. Thegradiometer has a main casing 61 formed from a twelve-sided ring 62 andhemispherical domes 63 (see FIG. 12). An internal mounting 5 isconnected to the ring 62. The ring 62 carries a support 65 to which afeed through flange 9 is coupled. A neck plug 11 formed of baffles 11 awhich sandwich foam 11 b is provided above the canister 3. The baffles11 a are supported on a hollow rod 93 which extends to the canister 3and which is also used to evacuate the canister 3.

With reference to FIG. 2 a first mount 10 of a rotatable mounting 5(FIG. 7) of the gradiometer is shown which comprises a base 12 and anupstanding peripheral wall 14. The peripheral wall 14 has a plurality ofcut-outs 16. The base 12 supports a hub 18.

FIGS. 3 and 4 show a second mount 20 which comprises a peripheral wall22 and a top wall 24. The peripheral wall 22 has four lugs 13 forconnecting the mount to the casing 61. The top wall 24 and theperipheral wall 22 define an opening 28. The peripheral wall 22 has afirst part 25, a second part 26 and a third part 27. The second mount 20is a monolithic integral structure and the first part 25 is formed bymaking a circumferential cut 19 through the peripheral wall except forthe formation of flexure webs as will be described hereinafter. Thethird part 27 is formed by making a second circumferential cut 29through the peripheral wall 22 except for flexure webs which will alsobe described hereinafter. The second mount 20 is mounted on the firstmount 10 by locating the hub 18 into the opening 28 and the lugs 13through respective cut-outs 16 as is shown in FIG. 7.

The first mount 10 is joined to the second mount 20. The first flexureweb 31 is formed in the first mount 10 so a primary mount portion of themount 10 can pivot about a web 31 relative to a secondary mount portionof the mount 10. This will be described in more detail with reference tothe second embodiment shown in FIGS. 13 to 21.

The lugs 13 connect the mounting 5 in the canister 3 which, in turn,locates in the Dewar 1 for cryogenic operation of the gradiometer.

The Dewar is in turn mounted in a first external platform for courserotational control of the gradiometer about three orthogonal x, y, xaxes. The mounting 5 mounts the sensor 40 (which will be described inmore detail hereinafter and which is preferably in the form of a massquadrupole) for much finer rotational adjustment about the x, y and zaxes for stabilising the gradiometer during the taking of measurementsparticularly when the gradiometer is airborne.

The first flexure web 31 allows the first mount 10 to move relative tothe second mount 20 about a z axis shown in FIG. 7.

FIGS. 5 and 6 are views along the lines IV and V respectively which inturn are along the cuts 19 and 29 shown in FIG. 3. The peripheral wall22 may be cut by any suitable cutting instrument such as a wire cutteror the like. FIG. 5 shows the bottom surface 19 a formed by the cut 27.As is apparent from FIGS. 3 and 5 the cut 27 has two inverted v-shapedpeaks 34. The apex of the peaks 34 is not cut and therefore form asecond flexure web 33 which join the first part 25 to the second part26. Thus, the second part 26 is able to pivotally rotate relative to thefirst part 25 about the x axis in FIG. 7. The second cut 29 is shown inFIG. 6 and again the bottom surface 29 a formed by the cut 29 isvisible. Again the second cut 29 forms two v-shaped peaks 35 and theapexes of the peaks 35 are not cut and therefore form a third flexureweb 37 which connect the second part 26 to the third part 27. Thus, thethird part 27 is able to pivotal rotate about the y axis shown in FIG.7.

FIG. 8 shows sensor 40 mounted on the mounting. The sensor 40 is anOrthogonal Quadrupole Responder—OQR sensor formed of a first mass and asecond mass in the form of a first bar 41 and a second bar 42 (not shownin FIG. 8) orthogonal to the bar 41 and which is of the same shape asthe bar 41.

The bar 41 is formed in a first housing 45 and the bar 42 is formed in asecond housing 47. The bar 41 and housing 45 is the same as bar 42 andthe housing 47 except that one is rotated 90° with respect to the otherso that the bars are orthogonal. Hence only the housing 45 will bedescribed.

The housing 45 has an end wall 51 and a peripheral side wall 52 a. Theend wall 51 is connected to rim 75 (FIGS. 2 and 7) of the wall 14 of thefirst mount 10 by screws or the like (not shown). The bar 41 is formedby a cut 57 in the wall 51 except for a fourth flexure web 59 whichjoins the bar 41 to the wall 51. The flexure web is shown enlarged inthe top view of the bar 41 in FIG. 9. Thus, the bar 41 is able to pivotrelative to the housing 45 in response to changes in the gravitationalfield. The bar 42 is mounted in the same way as mentioned above and alsocan pivot relative to its housing 47 in response to changes in thegravitational field about a fifth flexure web 59. The housing 47 isconnected to base 12 (FIG. 2) of the first mount 10.

The bar 41 and the housing 45 together with the flexure web 59 are anintegral monolithic structure.

Transducers 71 (not shown in FIGS. 2 to 6) are provided for measuringthe movement of the bars and for producing output signals indicative ofthe amount of movement and therefore of the measurement of thedifferences in the gravitational field sensed by the bars.

FIG. 10 is a schematic block diagram showing actuator control tostabilise the gradiometer by rotating the mounting 5 about threeorthogonal axes (x, y, z). A controller 50 which may be a computer,microprocessor or the like outputs signals to actuators 52, 53, 54 and55. The actuator 52 could rotate the mounting 5 about the x axis, theactuator 54 could rotate the mounting 5 about the y axis and theactuator 54 could rotate the mounting 5 about the z axis. However, inthe preferred embodiment, two of the four actuators 52, 53, 54 and 55are used to rotate the mounting about each axis so that rotation abouteach axis is caused by a combination of two linear movements providedfrom two actuators. The linear movement provided by each actuator willbe described with reference to FIGS. 31 and 32. The position of themounting 5 is monitored so that appropriate feedback can be provided tothe controller 50 and the appropriate control signals provided to theactuators to rotate the support 10 as is required to stabilise thesupport during movement through the air either within or towed behind anaircraft.

The preferred embodiment also includes angular accelerometers which aresimilar in shape to the bars 41 and 42 but the shape is adjusted forzero quadrupole moment. The linear accelerometers are simple pendulousdevices with a single micro pivot acting as the flexural hinge.

FIG. 11 is a view of a feedback control used in the preferredembodiment.

FIG. 12 is a cut away view of the gradiometer ready for mounting in theDewar 1 for cryogenic operation which in turn is to be mounted in theexternal platform. Although FIGS. 2 to 8 show the gradiometer with thebars 41 and 42 top and bottom, the instrument is actually turned on itsside (90°) so that the bars 41 and 42 are at the ends as is shown inFIG. 12.

FIG. 12 shows the mounting 5 arranged within the casing 61 and formed bythe ring 62 and the transparent hemispherical ends 63. The ring 22 hasconnectors 69 for connecting the internal wiring from transducers 71(see FIG. 8) and SQUID (Superconducting Quantum Interference Device)Electronics located in the casing 61 to the connectors 5 b (FIG. 1).

The transducers 71 measure the angle of displacement of the bars 41 and42 and the control circuitry (not shown) is configured to measure thedifference between them.

Error correction can be performed numerically based on digitised signalsfrom the accelerometers and a temperature sensor.

The transducers 71 are SQuID based transducers and the error correctionis made possibly by the large dynamic range and linearity of the SQuIDbased transducers.

FIGS. 13 to 21 show a second embodiment in which like parts indicatelike components to those previously described.

In this embodiment the first mount 10 has cut-outs 80 which effectivelyform slots for receiving lugs (not shown) which are connected to themount 10 in the cut-outs 80 and also to the second mount 20 shown inFIGS. 19 to 21. In this embodiment the lugs are separate components sothat they can be made smaller, and more easily, made than being cut withthe second mount section 20 which forms the second flexure web 33 andthe third flexure web 37.

In FIG. 13 a cut 87 is made to define the part 18 a of the hub 18. Thecut 87 then extends radially inwardly at 88 and then around centralsection 18 c as shown by cut 101. The cut 101 then enters into thecentral section 18 c along cut lines 18 d and 18 e to define a core 18f. The core 18 f is connected to the central section 18 c by theflexural web 31 which is an uncut part between the cut lines 18 e and 18d. The part 10 a therefore forms a primary mount portion of the mount 10which is separated from a secondary mount portion 10 a of the mount 10except for where the portion 18 a joins the portion 10 a by the flexuralweb 31. The part 18 a effectively forms an axle to allow for rotation ofthe part 18 a relative to the part 10 a in the z direction about theflexure web 31.

As is shown in FIG. 14, the cut line 88 tapers outwardly from the upperend shown in FIG. 14 to the lower end and the core 18 c tapers outwardlyin corresponding shape, as best shown in FIG. 17.

As is apparent from FIGS. 13 to 18, the first mount 10 is octagonal inshape rather than round, as in the previous embodiment.

FIGS. 19 to 21 show the second mount 20. FIG. 16 shows the second mount20 mounted in the first mount 10. As is best shown in FIGS. 19 and 20,the second mount 20 has cut-outs 120 which register with the cut-outs 80for receiving lugs (not shown). The lugs can bolt to the second mount 20by bolts which pass through the lugs and into bolt holes 121. The lugs(not shown) are mounted to the mount 20 before the mount 20 is securedto the first mount 10.

In the embodiment of FIGS. 19 and 20, the peaks 34 and inverted peaks 35are flattened rather than of V-shape as in the previous embodiment.

In this embodiment, top wall 24 is provided with a central hole 137 andtwo attachment holes 138 a. Three smaller holes 139 a are provided tofacilitate pushing of the housing 45 off the part 18 a if disassembly isrequired. When the second mount 20 is located within the first mount 10,the upper part of central section 18 c projects through the hole 137, asbest shown in FIG. 16. The mount 20 can then be connected to the mount10 by fasteners which pass through the holes 138 and engage in holes 139b (see FIG. 13) in the part 18 a.

Thus, when the first housing 45 and its associated bar 41 is connectedto the rim 75 of the housing 10 and the second housing 47 is connectedto the base 12, the housings 45 and 47 and their associated bars 41 and42 are therefore able to move about three orthogonal axes defined by theflexure web 31, the flexure web 33 and the flexure web 37.

As is best seen in FIG. 21 which is an exploded view of the three parts25, 26 and 27 which make up the second mount 20, an opening extendsthrough the mount 20 which is formed by the hole 137, hole 138 and hole139. It should be understood that the mount 20 shown in FIG. 21 is amonolithic structure and is merely shown in exploded view to clearlyillustrate the location of the flexural webs 33 and 35. Obviously theflexural web 33 shown in FIG. 21 joins with the part 26 and the flexuralweb 35 shown in FIG. 21 joins with the part 27. The holes 137, 138 and139 define a passage through which the axle or first portion 18 a of thefirst mount 10 can extend when the second mount 20 is located in thefirst mount 10.

Thus, when the second mount 20 is fixed to the part 18 a, the secondmount 20 can pivot with the first portion 10 a of the first mount 10about a z axis defined by the flexure web 31 whilst the second portionformed by the part 18 a remains stationary. Movement about the x and yaxes is achieved by pivotal movement of the second mount 20 about theflexure webs 33 and 35 as previously described.

FIG. 22 shows the linear and annular accelerometers 90 fixed to thehousings 45 and 47.

The gravity gradient exerts a torque on a rigid body with any massdistribution provided it has a non-zero quadrupole moment. For a planarbody, in the x-y plane and pivoted about the z-axis, the quadrupole isthe difference between moments of inertia in the x and y directions.Thus a square or circle has zero quadrupole moment, while a rectanglehas a non-zero value.

The torque produced is what constitutes the signal measured by thegradiometer.

There are two dynamical disturbances which can also produce torques andconsequently are sources of error.

The first is linear acceleration.

This produces a torque if the centre of mass is not exactly at thecentre of rotation—i.e. the bar is “unbalanced”. The bars 41 and 42 arebalanced as well as possible (using grub screws to adjust the positionof the centre of mass) but this is not quite good enough, so there is aresidual error. This error can be corrected by measuring the linearacceleration and using this to numerically subtract away the erroneouspart of the signal.

The second is angular motion.

There are two aspects to angular motion, each of which produces adifferent error.

The first is aspect angular acceleration.

Angular acceleration produces a torque on the mass distribution throughits moment of inertia (even if the quadrupole moment is zero). This isan enormous error and two preferred techniques are used to counteractit.

The first is to use internal rotational stabilization. This is depictedin the block diagram of FIG. 10. Here Ho(s) represents the sensorassembly pivoted about the mounting 5 (as per FIG. 9). The block A(s)represents the actuator, which provides the feedback torque to effectthe stabilization by canceling the applied disturbances. T(s) representsthe sensor (or transducer) which measures the effect of the applieddisturbance. This is the angular accelerometer. Using angularaccelerometers in rotational control is unusual—usually gyros and/orhighly damped tilt meters are used, but for our purpose the angularaccelerometers are better, as the error is proportional to the angularacceleration disturbance.

The second is to use common mode rejection CMRR—that is why 2 orthogonalbars are needed. For the two bars, the error torque produced by theangular acceleration is in the same direction, but the signal torqueproduced by the gravity gradient is in opposite direction.

Therefore, by measuring the difference in deflection between the twobars, the gradient is sensed but not the angular acceleration.

Therefore, two separate angular accelerometers 90 (labeled 90′ in FIG.22 for ease of identification) are provided. We have two independentoutput signals from the pair of OQR bars 41 and 42. The first isproportional to the difference in deflection, which gives the gradientsignal and the second is proportional to the sum of their deflections,which is proportional to the angular acceleration and provides thesensor for the z-axis rotational control.

The x and y axes require separate angular accelerometers. Rotationalstabilization about these axes is required because the pivot axes of thetwo bars are not exactly parallel and also to counteract the second formof error produced by angular disturbance, discussed below.

The second aspect is angular velocity.

Angular velocity produces centrifugal forces, which are also a source oferror. The internal rotational stabilization provided by the actuatorsreduces the angular motion so that the error is below 1 Eotvos.

FIG. 23 shows main body 61 and connector 69 with the hemispherical endsremoved.

FIG. 24 is a plan view of housing 45 according to a still furtherembodiment of the invention. As is apparent from FIG. 24, the housing 45is circular rather than octagonal, as is the case with the embodiment ofFIG. 8.

The housing 45 supports bar 41 in the same manner as described viaflexure web 59 which is located at the centre of mass of the bar 41. Thebar 41 is of chevron shape, although the chevron shape is slightlydifferent to that in the earlier embodiments and has a more rounded edge41 e opposite flexure web 59 and a trough-shaped wall section 41 f, 41 gand 41 h adjacent the flexure web 59. The ends of the bar 41 havescrew-threaded bores 300 which receive screw-threaded members 301 whichmay be in the form of plugs such as grub screws or the like. The bores300 register with holes 302 in the peripheral wall 52 a of the housing45. The holes 302 enable access to the plugs 301 by a screwdriver orother tool so that the plugs 301 can be screwed into and out of the bore300 to adjust their position in the bore to balance the mass 41 so thecentre of gravity is at the flexure web 59.

As drawn in FIG. 24, the bores 300 are a 45° angle to the horizontal andvertical in FIG. 24. Thus, the two bores 302 shown in FIG. 24 are atright angles with respect to one another.

FIG. 24 also shows openings 305 for receiving the transducer 71 formonitoring the movement of the bar 41 and producing signals which areconveyed to the SQUID device. Typically, the transducer is in the formof a coil and as the bar 41 moves slightly due to the gravity differenceat ends of the bar, a change in capacitance occurs which alters thecurrent in the coil to thereby provide a signal indicative of movementof the bar 41.

FIG. 25 is a more detailed view of part of the housing of FIG. 24showing the openings 305. As can be seen from FIG. 25, the openings 305have shoulders 401 which form grooves 402. A spring 403 is arrangedadjacent surface 406.

FIG. 26 shows the transducer 71. The transducer 71 is formed by agenerally square macor plate 410 which has a circular boss 407. A coil408 is wound about the boss 407 and may be held in place by resin or thelike. The coil 408 may be multi-layer or a single layer coil.

FIG. 27 shows the location of the plate 410 in the opening 305 in whichthe plate locates in the grooves 402 and is biased by the spring 403against the shoulders 401 to hold the plate 410 in place with the coils408 being adjacent the edge face 41 a of the bar 41.

Thus, the coil 408 and the bar 41 form an 1c circuit so that when thebar 41 moves, the current passing through the coil 408 is changed.

As will be apparent from FIG. 24, four transducers 71 are arrangedadjacent the ends of the bar 41. The other housing 47 also has fourtransducers arranged adjacent the bar 42. Thus, eight transducers 71 areprovided in the gradiometer.

FIG. 28 is a diagram of the bars 41 and 42 showing them in their “inuse” configuration. The transducers which are located in the openings305 are shown by reference numbers 71 a to 71 e to equate to the circuitdiagrams of FIGS. 29 and 30.

With reference to FIGS. 29 and 30, transducers 71 a and 71 b associatedwith the bar 41, and transducers 71 g and 71 e associated with the bar42 are used to provide the gravity gradient measurements.

Input terminals 361 provide input current to the superconductingcircuits shown in FIG. 29. Heat switches which may be in the form ofresistors 362 are provided which are used to initially set thesuperconducting current within the circuit. The heat switches 362 areinitially turned on for a very short period of time to heat those partsof the circuit at which the resistors 362 are located to stop thoseparts of the circuit from superconducting. Currents can then be imposedon the superconducting circuit and when the heat switches formed by theresistors 362 are switched off, the relevant parts of the circuit againbecome superconducting so that the current can circulate through thecircuits subject to any change caused by movement of the bars 41 and 42under the influence of the gravity gradient and angular acceleration, aswill be described hereinafter.

The transducers 71 a, 71 b, 71 g and 71 e are connected in parallel tocircuit line 365 and to circuit line 366 which connect to a SQUID 367.

Thus, as the bars 41 and 42 rotate about their respective flexure web,the bars 41 and 42, for example, come closer to the transducer 71 a andtherefore further away from the transducer 71 b, and closer to thetransducer 71 h and further away from the transducer 71 g respectively.This therefore changes the current flowing through the transducers andthose currents are effectively subtracted to provide signals forproviding a measure of the gravity gradient.

As is shown in FIG. 31, transducers 71 c and 71 d form a separatecircuit and are used for frequency tuning of the bar 41 and transducers71 a and 71 b. Similarly, the transducers 71 e and 71 f are used forfrequency tuning of the bar 42 and the transducers 71 g and 71 h.Frequency tuning of the bars is important because the bars should beidentical in order to reject angular accelerations. The frequency tuningcircuits therefore enable electronic tuning of the bars to matchresonant frequencies and to achieve mode rejection so that each of thebars does function in an identical manner.

The transducers 71 a, 71 b, 71 g and 71 h are also used to form angularaccelerometers for measuring the angular movement of the mounting 5 sothat feedback signals can be provided to compensate for that angularmovement.

To do this, the line 366 is connected to a transformer 370. The polarityof the signals from the transducers 71 a and 71 b and 71 g and 71 h arereversed so that the output of the transducer 370 on lines 371 and 372is an addition of the signals rather than a subtraction, as is the casewhen the gradient is measured so the addition of the signals gives ameasure of the angular movement of the bars. The outputs 371 and 372 areconnected to SQUID device 375 for providing a measure of the angularacceleration which can be used in the circuit of FIG. 10 to providecompensation signals to stabilise the mounting 5.

Thus, according to the preferred embodiment of the invention, theangular accelerometers 90′ provide a measurement of angularacceleration, for example, around the x and y axes, and the angularaccelerometer formed by the bars 41 and 42 and the transducers 71 a, 71b, 71 g and 71 h provide a measure of the angular accelerometer aroundthe, for example, z axis.

FIGS. 31 and 32 show an actuator for receiving the control signals toadjust the mounting in response to angular movement of the mounting 5.

The actuator shown in FIGS. 31 and 32 are schematically shown in FIG. 10by reference numerals 52, 53, 54 and 55. All of the actuators are thesame and FIGS. 31 and 32 will be described with reference to theactuator 52 which makes adjustment around the x axis shown in FIG. 10.

Actuator 52 shown in FIG. 31 has a hollow disc housing 310 which has amounting bracket 311 for connecting the disc housing 310 to mounting 5.The hollow disc housing 310 therefore defines an inner chamber 312 inwhich is located coil support plate in the form of a disc 313. The disc313 has a wide hub section 314 and two annular surfaces 315 and 316 ontowhich windings W1 and W2 of coils are wound about the hub 314.

The disc 313 is also provided with a radial bore 319 and a hole 320 atthe periphery of the disc 313 which communicates with the bore 319. Ahole 321 is provided at the hub 314 and communicates with the bore 319and extends to a hollow rod 328 which locates in a tube 330. The rod 330is fixed to the disc 313 and also to support frame 340 which is fixed tomain body 61 (not shown in FIG. 31). The tube 330 is connected to thedisc housing 310 for movement with the disc housing 310 relative to disc313, rod 328 and frame 340.

The winding W1 provided on the face 315 has a lead 331 which passesthrough the hole 320 and then through the bore 319 to the hole 321 andthen through the tube 328 to the right, as shown in FIG. 31. A lead 332from the other end of the winding W1 passes through the hole 321 andthrough the hollow rod 328 also to the right so that current can besupplied to the winding W1 through the leads 331 and 332.

The second winding W2 provided on the face 316 has a lead 333 whichpasses through a radial hole 334 and bore 345 in the disc 313 and thenthrough hole 337 to tube 328 and to the left in FIG. 31. The other endof the winding W2 has a lead 338 which passes through the hole 337 intothe tube 328 and to the left in FIG. 31. Thus, current can circulatethrough the winding W2 via the leads 333 and 338.

When the windings W1 and W2 are energised or the current passing throughthe windings changes, the disc housing 310 is moved relative to the disc313 and frame 340 and because the disc housing 310 is connected to themounting 5 by the bracket 311, the mounting 5, in the case of theactuator 52, is adjusted. The movement of the disc housing 310 isgenerally a longitudinal movement (i.e. linear movement) in thedirection of the axis of the tube 330 and rod 328. To facilitate suchmovement, clearance is provided between the ends of the rod 330 and theframe 340 and about the disc 313. The bracket 311 is offset relative tothe flexure web (such as the flexure web 37) so that movement of thehousing 310 applies a torque to the first part 25 of the mounting 5 tocause rotation of the part 25 about the flexure web 37.

In the preferred embodiment of the invention, four actuators areprovided for providing actual adjustment about the various axes andflexure webs and the actuators operate in combination in response tosignals received from the angular accelerometers to maintain stabilityof the mounting 5 when the gradiometer is in use.

For cryogenic operation of the gradiometer, the mounting 5, housings 45and 47, bars 41 and 42, the hollow disc housing 310, coils, andelectrical leads referred to previously, are all made fromsuperconducting material such as niobium.

In embodiments of the invention where the gradiometer is notcryogenically operated, the components can be formed from othermaterials such as aluminium.

The angular accelerometers 90′ have zero quadrupole moment which meansthat the centre of mass coincides with the flexure web and thatconsequentially they are insensitive to both gravity gradient andcentrifugal force. Linear accelerometers 90″ (FIG. 22) could also beprovided. The linear accelerometers 90″ do not apply active compensationbut may apply corrections to the final measured gradient data. Thus,data relating to linear acceleration can be recorded and possibly usedin later processing.

One or both of the bars 41 and 42 can also be used as an angularaccelerometer to provide a measure of angular movement of the mounting 5so that appropriate feedback signals can be generated to compensationfor that movement by control of the actuators previously described.

In the preferred embodiment, four angular accelerometers are providedwith two of the accelerometers being formed by the bars 41 and 42. Theuse of four accelerometers arranged at 45° angles with respect to oneanother enables adjustment about the x, y and z axes by torque suppliedfrom two or more of the actuators at any one time.

The disc 310 prevents flux from the windings W1 and W2 from leaving theactuator and because the leads 331 and 332 and 333 and 338 leave theactuator through the elongate tube 330, the ability of flux to pass outof the actuator is substantially prevented.

Thus, spurious magnetic fields which may detrimentally effect operationof the instrument are not generated by the actuator and therefore do notinfluence the sensitivity or operation of the instrument.

The tube 330 preferably has a length to diameter ratio of 10:1 at theleast.

The disc plate 316 is preferably formed from macor and the hollow dischousing 310 is formed in two parts 310 a and 310 b. The part 310 bforming a closure panel which enables the disc 313 to be located in thechamber 312 and then the disc housing 310 closed by locating the plate310 b in place.

With reference to FIGS. 33 and 34, the manner in which the balance ofthe bars 41 and 42 is achieved will be described. A pair of displacementsensors formed by capacitors 400 and 401 are provided for two mainpurposes:

-   1. To measure the residual linear acceleration sensitivity of each    bar 41 (and 42) to enable the bars to be mechanically balanced using    the grub screws 301 described with reference to FIG. 24, before    operation at low temperatures; and-   2. To measure the induced linear acceleration sensitivity of each    bar 41 and 42.

The bars 41 and 42, in their respective housings, are rotated in a jig(not shown) through 360°. This provides an acceleration range of 2g_(E), which is typically 100 times greater than the accelerations whichmay be conveniently applied at low temperature. A typically requirementis for the capacitors 400 and 401 to be able to detect 0.1 nm over aperiod of 1 to 20 minutes. A pair of capacitors 400 and 401 is requiredfor each bar to provide some discrimination against sensor drift, sincerotation of the bar 41 will cause one capacitor 400 to increase and theother capacitor 401 to decrease by the same amount, as is shown in FIG.33, whereas thermal expansion will cause both outputs of the capacitors400 and 401 to increase. The capacitors 400 and 401 remain in place,even though they are unusable at low temperatures, and therefore theircomponents need to be non-magnetic so as to not interfere with theoperation of the gradiometer and, in particular, its nearbysuperconducting circuitry.

FIG. 33 shows that as the bar 41 pivots, the gap applicable to thecapacitor 400 decreases and the gap of the capacitor 401 increases.

The capacitors 400 and 401 are formed by the face 41 a of the bar 41(and the corresponding face on the other bar 42) and second plates 405which are spaced from the face 41 a. The gap between the plates of therespective capacitors 400 and 401 must typically be resolved to about 1ppm.

FIG. 34 shows the calibration circuit applicable to the capacitor 400. Acircuit for the other capacitor 401 is identical.

The capacitor 400 forms a high Q-factor resonant circuit with inductor410. The inductor 410 and capacitor 400 are provided parallel tocapacitors 411 and 412 and connect via capacitor 413 to an amplifier414. The output of the amplifier 414 is provided to a frequency counter415 and also fed back between the capacitors 412 and 411 by line 416.The capacitor 400 therefore determines the operating frequency of theamplifier 414 which can be read to a high precision.

If the bar 41 is out of balance, the frequency counter 45 will tend todrift because of the imbalance of the bar. This can be adjusted bymoving the grub screws 301 into and out of the masses as previouslydescribed until balance takes place. The amplifier 414 can then bedisconnected from the frequency counter 415 so that the gradiometer canbe arranged within the Dewar 1 with the other parts of the circuitsshown in FIG. 34 in place.

Since modifications within the spirit and scope of the invention mayreadily be effected by persons skilled within the art, it is to beunderstood that this invention is not limited to the particularembodiment described by way of example hereinabove.

In the claims which follow and in the preceding description of theinvention, except where the context requires otherwise due to expresslanguage or necessary implication, the word “comprise” or variationssuch as “comprises” or “comprising” is used in an inclusive sense, i.e.to specify the presence of the stated features but not to preclude thepresence or addition of further features in various embodiments of theinvention.

1. A gravity gradiometer for measuring components of the gravity gradient tensor, comprising: an external platform for mounting a Dewar; a sensor arranged in the Dewar for cryogenic operation for measuring the components of the gravity gradient tensor; an internal mounting for mounting the sensor in the Dewar; a first feedback controller for monitoring a disturbance of the external platform and producing a force to counteract movement of the external platform to stabilise the external platform; and an internal feedback controller for monitoring a disturbance of the internal platform caused by any movement of the external platform after stabilisation of the external platform to counteract movement of the internal platform and stabilise the sensor.
 2. The gravity gradiometer of claim 1 wherein movement of the internal platform caused by the disturbance is sensed by linear and angular accelerometers to provide feedback signals to the controller.
 3. The gravity gradiometer of claim 1 wherein the internal mounting comprises a mounting for mounting the sensor for movement relative to three orthogonal axes and the controller is for supplying signals to actuators for moving the mounting about any one or more of the three orthogonal axes to stabilise the mounting and therefore the sensor. 